The present invention relates to data transmissions over a telecommunications network, and more particularly to the transmission of data in a cellular telecommunications network. The invention further relates to a device utilising that method.
In telecommunications networks such as GSM (Group System for Mobile Communication) and UMTS (Universal Mobile Telecommunication System) and UTRAN (Universal Telecommunication Radio Access Network), in which WCDMA (Wideband Code Division Multiple Access) is one radio transmission method, the transferred data, e.g. speech data, is compressed before it is transported over the radio interface. This reduces the bandwidth demands on the scarce resource radio interface. To achieve this compression, codecs (coder and decoder algorithms as well as a means provided for performing that algorithm) are used. As these codecs do not work lossless, the number of transcoding stages within the telecommunications core network has to be minimized in order to maximize data quality. In the following data quality in the meaning of this application is the quality of audio or video data.
In GSM-type networks the BSS (Base Station sub-System), specifically the TRAU (Transcoding and Adaption Unit), termed as “transcoder unit” in the following, is responsible of converting the compressed speech to PCM (Pulse Code Modulation), which is then used on the A-interface towards the core network.
In UTRAN-type networks the responsibility of converting the compressed speech to PCM lies in the telecommunications core network, specifically the MGW (Media Gateway). Therefore the point where in the core network this conversion is done can be negotiated amongst the involved call control nodes. This negotiation is done using OoBTC-procedures (Out of Band Transcoder Control). In the ideal case of a call between two UTRAN terminals, compressed speech is transported end-to-end between the terminals without any additional transcoding on the path. This is called Transcoder Free Operation (TrFO). TrFO allows for substantially reduced bandwidth demands in the core network and achieves optimal speech quality. A detailed description can be found in the technical specification 3GPP TS 23.153.
For GSM access a similar mechanism can be used called Tandem Free Operation (TFO). In a call between two GSM terminals two transcoder units in the BSS are involved. After the call is established, these two transcoder units negotiate via inband messages by stealing bits from the PCM stream. If compatible codecs are used on both sides, compressed speech can be exchanged between both transcoder units. TFO does not reduce the bandwidth demands on the core network but achieves the same optimal speech quality. More information can be found in the technical specification3GPP TS 28.062.
The two mechanisms, TFO for GSM and OoBTC/TrFO for UMTS and UTRAN, have been harmonized and can be combined in order to improve the quality of calls between GSM and UTRAN terminals.